This invention relates to a machine and method for dynamically determining the unbalance of unbalanced rotors, especially wheels of motor vehicles.
It is conventional to determine rotational unbalance of a body such as a vehicle wheel by attaching the body to a rotatable shaft, driving the shaft at a constant velocity with the body attached and simultaneously measuring the vibration of the shaft brought about by the unbalance of the body. These vibrations can then be related to the unbalance by methods well known to the art. In most modern balancing machines, the measurement of vibration is accomplished by means of transducers which produce electrical signals that are proportional to the magnitude of vibration; which electrical signals can be converted to a readable measure of unbalance.
Present commercial wheel balancers measure unbalance with their drive motor engaged to and driving the balancing shaft. This offers the advantage of providing a constant rotational velocity. However, the engaged drive unit introduces vibrations of its own and by its bulk dampens the vibrations of the unbalanced body so that high angular velocities are required to detect them. The variety of belt, gear and direct drive systems that have been proposed in attempts to minimize these shortcomings attest to their seriousness.
U.S. Pat. No. 3,076,342 issued to Karl Hilgers of the Losenhausenwerk Dusseldorfer Maschinebau A.G. discloses a balancing machine for small rotating bodies in which a drive unit spins the body and is then disconnected. The unbalance is measured as the body coasts to a stop with the measured value being recorded by a mechanical meter lock which engages at a predetermined speed. Unfortunately, with its continuous measurement, this device essentially averages unbalance over a range of speeds. With constant sized objects, such as might be found in a machine shop setting, there is a uniform rate of deceleration and this averaging effect can be compensated for. In a common wheel and tire balancing setting, the objects to be balanced vary in size from as little as 20-30 lbs to as much as 300-400 lbs. In addition, the tread patterns of tires vary from very smooth to extremely coarse. This leads to widely differing wind resistances. These factors give rise to widely variable deceleration rates which cannot be routinely compensated for and would require time-consuming repeat spins to obtain an accurate unbalance reading.
Our studies have disclosed another problem as well when a coasting balancing system is employed with widely variable tires and wheels. It has been observed that it takes a finite time for the vibrations introduced by decoupling the drive unit to settle out, i.e., dampen out. In addition, electrical circuits require a similar settling period. This time period is on the order of 2-4 seconds. If the object being measured has a very rapid deceleration so that the preset measuring velocity is reached too soon after decoupling, that is before the vibrations and electrical circuiting have settled, an inaccurate reading results. This can occur with very heavy tread, high wind resistance tires. Merely spinning all tires to a higher speed is not a uniform answer as this results in unacceptably long coast periods with smoother, lower wind resistance tires.
The present invention has as its primary object the elimination of the disadvantages of the prior art balancing machines and the solution of these problems that have been uncovered.